logging: remove CONFIG_LOG2_MSG_PKG_ALWAYS_ADD_RO_STRING_IDXS

CONFIG_LOG2_MSG_PKG_ALWAYS_ADD_RO_STRING_IDXS was added as
a stop-gap measure to support Sys-T catalog messages. Since
the Sys-T backend has envolved to use tagged argument, this
kconfig and its effect are no longer needed. So remove it.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>